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Kepler's Laws of Planetary Motion

Kepler's law of areas

HowStuffWorks

For centuries, scientists battled with one another and with religious leaders about the planets' orbits, especially about whether they orbited our sun. In the 16th century, Copernicus put forth his controversial concept of a heliocentric solar system, in which the planets revolved around the sun -- not the Earth. But it would take Johannes Kepler, building on work performed by Tyco Brahe and others, to establish a clear scientific foundation for the planets' movements.

Kepler's three laws of planetary motion -- formed in the early 17th century -- describe how planets orbit the sun. The first law, sometimes called the law of orbits, states that planets orbit the sun elliptically. The second law, the law of areas, states that a line connecting a planet to the sun covers an equal area over equal periods of time. In other words, if you're measuring the area created by drawing a line from the Earth to the sun and tracking the Earth's movement over 30 days, the area will be the same no matter where the Earth is in its orbit when measurements begin.

The third one, the law of periods, allows us to establish a clear relationship between a planet's orbital period and its distance from the sun. Thanks to this law, we know that a planet relatively close to the sun, like Venus, has a far briefer orbital period than a distant planet, such as Neptune.

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